Sol-gel chemistry of transition metal oxides

Acid sulfate waters are produced mostly by the oxidation of common sulfide minerals such as pyrite, chalcopyrite, pyrrhotite, and marcasite in rocks, soils, sediments, and industrial wastes. This spontaneous process of mineral weathering plays a fundamental role in the supergene alteration of ore deposits, the formation of acid sulfate soils, and the mobilization and release of acidity and metals to surface and ground waters. The purely natural process of “acid rock drainage” is often intensified by human activities related to mining, mineral processing, construction, soil drainage, and dredging. Geochemical reaction rates are accelerated because physical disturbance gives greater exposure of mineral surfaces to air and water, and to microbes that catalyze the reaction process. Large quantities of reactive sulfides are also concentrated and exposed to air as a result of mining and mineral processing. Acid sulfate waters produce a number of fairly insoluble hydroxysulfate and oxyhydroxide minerals that precipitate during oxidation, hydrolysis, and neutralization. The objective of this chapter is to describe the formation, properties, fate, and environmental implications of the nano- to microphase hydroxy-sulfates of Fe and Al that are precipitated from acid sulfate waters. These minerals are commonly of poor crystallinity and difficult to characterize. Much remains to be learned about their occurrence, formation, and properties.

### Mine drainage

The best known examples of acid sulfate waters are those released from mines where coal and metallic sulfide ores have been exploited (Ash et al. 1951, Barton 1978, Nordstrom 1982a, Rose and Cravotta 1998, Nordstrom and Alpers 1999). There may be as many as 500,000 inactive or abandoned mine sites in the United States alone (Lyon et al. 1993). Although most of these pose no immediate water-quality problem, Kleinmann (1989) estimated that about 19,300 km of streams and more than 72,000 ha of lakes and reservoirs have been …

Iron and Aluminum Hydroxysulfates from Acid Sulfate Waters

From Fe(III) Ions to Ferrihydrite and then to Hematite

Phase transformations of iron oxides, oxohydroxides, and hydrous oxides in aqueous media

The self-assembled synthesis of a hierarchical graphene oxide (GO)-carbon nanotubes (CNTs)-α-FeOOH decorated composite aerogel (α-FeOOH@GCA) through a facile in-situ hydrolysis route is reported for the first time and the materials was tested for its performance as Fenton-like catalyst. The introduction of GO-CNTs clearly mediated the morphology to aligned α-FeOOH nanorods ( ca. 100 nm) within aerogel matrix comparing with pristine urchin-like α-FeOOH three-dimensional microstructures ( ca. 1 μm). This three dimensional porous aerogel network provided efficient charge/mass-transfer leading to great enhancement of the catalytic activity of α-FeOOH. The outstanding catalytic performance of this composite in degradation of organics with different charge and structure, i.e. Orange II (OII), rhodamine B (RhB), methylene blue (MB), phenol and endocrine disruptor bisphenol A (BPA) was demonstrated. For example, the discoloration of OII with pseudo first-order rate constant of 0.10 min −1 significantly exceeded that of pristine α-FeOOH. At relatively low concentration of α-FeOOH@GCA catalyst (125 mg L −1 ) and H 2 O 2 (0.55 mM) showed excellent catalytic activity for efficient (∼99%) discoloration of OII (40 mg L −1 ) under a 60 min UV 365 irradiation in the pH range 3–10. The different charge of five target contaminats greatly determined the surface-catalyzed degradation kinetics at natural solution pH in the order of cationic > neutral > anionic organics due to the negatively charged carbon-based aerogel matrix. The new identified desulfonation intermediates elucidated through UPLC–MS analysis indicated two reaction pathways: 1) hydroxylation and 2) desulfonation by-products followed by cleavage of the azo bond as the predominant degradation pathway of OII. The elimination of the acute toxicity of the parent contaminant to luminescent bacterium Q67 was consistent with the formation of less toxic degradation by-products identified. Free radical quenching studies accessed the role of hydroxyl radical ( OH), superoxide anion radical (O 2 − ) and singlet oxygen ( 1 O 2 ) as the dominating reactive oxygen species (ROS). The quantitative studies to measure radical concentrations using relative molecular probes showed the effective activation of H 2 O 2 led to high rate production of ROS accounted for OII degradation. The greatly enhanced photocatalytic property of this hybrid was correlated with the efficient conversion between Fe 2+ /Fe 3+ and synergistic coupling between α-FeOOH and carbon-based aerogel matrix evidenced through the formation of Fe O C chemical bonds was verified by X-ray photoelectron (XPS) analysis. Based on its simple and scalable preparation route as well as its excellent UV 365 or visible light-responsive catalytic performance, this hybrid exhibits a high potential to be used as an efficient and environmental-friendly catalyst for water remediation.

D. den Ottelander

Papers

The influence of sulfate ions on the formation of iron(III) oxides